There are frequencies for cellular phones in North America, namely the PCS (Personal Communications Service) band and the cellular band. In the PCS band, a frequency band from 1700 MHz to 2200 MHz is used as the 2 GHz band. In the cellular band, a frequency band from 820 MHz to 960 MHz has been previously used as the 800 MHz band, and recently, a mobile telecommunications service based on a communication standard called LTE (Long Term Evolution) has started as the 700 MHz band in the cellular band.
In the United States, Verizon Wireless Inc. and AT&T Inc. offer wireless data communication services using LTE. Verizon Wireless Inc. uses a frequency band from 747 MHz to 787 MHz, and AT&T Inc. uses a frequency band from 704 MHz to 746 MHz. Cellular phones or smart phones have only to be equipped with an antenna adapted to a frequency band of either of the companies, whereas it is desired that notebook computers (hereinafter called laptop PCs) and other types of mobile computing devices, including tablets, netbooks, and ultra laptops be equipped with an antenna adapted to a range of frequencies from 704 MHz to 787 MHz to cover the frequency bands of both companies in order to use the frequency band for cellular phones in the United States.
Japanese Patent No. 4121799 discloses a dual band antenna composed of an exciter and two quarter-wavelength antennas. The exciter is composed of a dipole antenna resonating with a fundamental frequency and a harmonic resonance frequency. One quarter-wavelength antenna is an inverted-L dipole antenna resonating with the fundamental frequency and the other quarter-wavelength antenna is an inverted-L dipole antenna resonating with an n-order harmonic resonance frequency. The open end of the one quarter-wavelength antenna is electrostatically coupled to the exciter in a position in which the current distribution of the fundamental frequency is minimized, and the open end of the other quarter-wavelength antenna is electrostatically coupled to the exciter in a position in which the current distribution of the n-order harmonic resonance frequency is minimized.
Japanese Patent Application Publication No. 2010-288175 discloses a multiband antenna of a T monopole structure composed of a common conductor and two horizontal conductors different in length. This antenna has the common conductor and the respective horizontal conductors form a quarter-wavelength radiation conductor to resonate with two frequencies and operate in a serial resonance mode. With this antenna, it is described that the low frequencies adapt to the 800 MHz band.
Japanese Patent Application Publication No. 2007-214961 discloses a multiband antenna apparatus capable of reducing electrostatic coupling among multiple antennas. A support base member includes a flat face portion and peripheral end faces orthogonal to the flat face portion. A first antenna element and a second antenna element are branched from the same power feed point. The first antenna element is laid out along the peripheral end faces and the second antenna element is provided along the peripheral end face of the flat face portion. The distal ends of the antenna elements are arranged orthogonal to each other at positions where the distal ends do not face each other. With this antenna, it is described that the low frequencies adapt to the 800 MHz band.
Japanese Patent Application Publication No. 2009-135633 discloses an antenna in common use with multiple frequencies for mobile terminals, composed of an inverted-L driven element, a first radiation conductor, and a second radiation conductor. The first radiation conductor having one folded portion and the second radiation conductor having two folded portions are branched in opposite directions from a common conductor connected to the ground. The first radiation conductor is arranged partially in parallel with a horizontal portion of the driven element and capacitively coupled. The second radiation conductor is arranged partially in parallel with the first radiation conductor and capacitively coupled. With this antenna, it is described that low frequencies adapt to the 900 MHz band.
“A Study of Broadband Monopole Antenna with Parasitic Elements,” Sugimoto, et. al., 2008 IEICE Tokyo Branch Student Research Conf. discloses an asymmetric monopole antenna having a size incorporable in a mobile terminal as shown in FIG. 6A and ultrawideband characteristics. This antenna is composed of a T-shaped feed element and inverted-L parasitic elements respectively having branch conductors parallel to each other on both sides of a common portion of the feed element. Use of the parasitic elements results in exciting two new resonances in a high frequency range, achieving a wider bandwidth from 1.9 GHz to 5 GHz.
Since the antenna increases in length as the resonance frequency decreases, a large space is required to accommodate low frequencies. An inverted-F dual-band antenna as shown in FIG. 6B has been mounted in traditional laptop PCs. The laptop PCs are required to incorporate multiple antennas in a display case for wireless communication, such as WLAN and WiMAX, in addition to cellular phone lines. However, when the antenna structure of FIG. 6B is adopted for a new antenna to adapt the low-frequency side to the 700 MHz band, the length of each element increases, causing a problem that it cannot be accommodated in the limited space.
Further, in order to incorporate a new antenna in a laptop PC, it is necessary to achieve a wide bandwidth capable of covering the frequency bands of both companies in the United States within the limits of space given to traditional antennas. It is also necessary to mount multiple antennas in a small space in a laptop PC, and this may not be able to secure enough distance therebetween depending on the mounting condition. Therefore, there is a need for the new antenna to have a structure that is not likely to cause radio wave interference with other antennas.
The dual band antenna described in Japanese Patent No. 4121799 requires a space equal to or larger than the sum of the lengths of the horizontal portions of the two quarter-wavelength antennas in the longitudinal direction of the antenna pattern. In the T-shaped asymmetric monopole antenna described in “A Study of Broadband Monopole Antenna with Parasitic Elements,” Sugimoto, et. al., 2008 IEICE Tokyo Branch Student Research Conf., since the open ends of the two inverted-L parasitic elements extend out in opposite directions, a large space is also required in the longitudinal direction of the antenna pattern. Further, since none of the antennas described in the above-mentioned related art documents conforms to the 700 MHz band, there is a need to develop an antenna having a new structure capable of being accommodated in a small space provided in a laptop PC.